1. Field of the Invention
The present invention relates to an improved apparatus and process for extracting high-purity aromatics from gasoline which increases system capacity while reducing energy consumption.
2. Prior Art
Several commercially proven processes and apparatus are available for extracting high-purity aromatics from gasoline, coke oven light oil, and pyrolysis naphtha. Most include a liquid-liquid extractor followed by an extractive distillation column for extracting a high-purity aromatic stream, apparatus for removing solvent from the product streams, and solvent conditioning facilities.
The extraction of benzene and heavier aromatic homologs has been practiced commercially for about a century. Prior to the preparation of high-octane gasoline from crude oil, aromatics were extracted from liquids produced during the coking and gasification of coal. With the advent of platinum reforming ("Platforming") in the late 1940's, a large source of less expensive aromatics became available in oil refineries.
At about the same time Dow and others were developing commercial plants to produce ethylene glycol for the automotive antifreeze market. One of the heavy byproducts of this process was di-ethylene glycol. Dow found that this material could be used to extract aromatics from gasoline, and developed a process to accomplish this.
Dow made an arrangement for UOP to market the process once it was proved, naming it UDEX in honor of the new partnership promoting the process. This process dominated the extraction field through the 1950's until the Shell Sulfolane process supplanted it in the 1960's.
The early UDEX units used di-ethylene glycol ("DEG") and diglycol amine ("DGA") for solvents. The consumption of energy was typically in the range of 1200 to 1500 BTU/pound of extract. In the early 1960's, tri-ethylene glycol replaced most of the DEG/DGA, reducing energy consumption to 1000 to 1200 BTU/pound of extract, and increasing unit capacity by 20 to 30%. In the 1970's, tetra-ethylene glycol replaced most of the tri-ethylene. With this change, the energy consumption dropped to the range of 800 to 1000 BTU/pound of extract and the capacity increased another 10 to 20%. A solvent additive called "Carom" was used in the 1960's to decrease the energy consumption and increase capacity, each changing in the range of 5 to 10%.
The introduction of the Shell Sulfolane process in the 1960's ended the design and construction of most UDEX apparatus. The Shell Sulfolane apparatus usually consumes less than 700 BTU/pound of extract. While this is a strong advantage, the process has two important disadvantages.
First, the Sulfolane process requires four large columns rather than the two required for the UDEX process. This increases capital cost.
Second, the solvent can become corrosive. Reboiler replacement and exotic metallurgy are not uncommon due to this corrosive nature. Entire columns have had to be replaced at times.
Thus, because of the low capital cost and non-corrosive nature of the glycol units, a UDEX apparatus having a low consumption of energy would have substantial application in the aromatics field.